Hitherto, in the application field of tires for which a wear resistance and a mechanical strength are required, there have been extensively used rubber compositions that are enhanced in mechanical strength by incorporating a reinforcing agent such as carbon black in a rubber component such as a natural rubber and a styrene-butadiene rubber.
It is known that the carbon black exhibits its reinforcing effect by physically or chemically adsorbing the aforementioned rubber component onto a surface of respective particles of the carbon black.
However, when the particle size of the carbon black used in the rubber composition is as large as from about 100 to about 200 nm, it is generally difficult to attain a sufficient interaction between the carbon black and the rubber component, so that the resulting rubber composition tends to be hardly improved in mechanical strength to a sufficient extent.
In addition, tires produced from such a rubber composition tend to exhibit a low hardness and therefore tend to be insufficient in steering stability.
On the other hand, when the carbon black used in the rubber composition has an average particle size as small as from about 5 to about 100 nm and therefore a large specific surface area, the resulting rubber composition can be improved in properties such as mechanical strength and wear resistance owing to a large interaction between the carbon black and the rubber component.
In addition, tires produced from such a rubber composition can be improved in steering stability owing to an increased hardness thereof.
However, in the case where the carbon black having such a small average particle size is used in the rubber composition, it is known that the resulting rubber composition tends to be deteriorated in dispersibility of the carbon black therein owing to a high cohesive force between the carbon black particles.
The deteriorated dispersibility of the carbon black in the rubber composition tends to induce a prolonged kneading step and therefore tends to give an adverse influence on productivity of the rubber composition.
Also, the deteriorated dispersibility of the carbon black tends to cause generation of heat in the rubber composition, so that tires produced therefrom tend to be deteriorated in rolling resistance performance and may frequently fail to satisfy the requirements for low rolling resistance tires (so-called low-fuel consumption tires).
Further, in the case where the carbon black used in the rubber composition has a small average particle size, there tends to occur such a problem that the resulting rubber composition exhibits a high viscosity and therefore is deteriorated in processability.
Thus, the mechanical strength and hardness of the rubber composition for tires are properties having a contradictory relation with the rolling resistance performance and processability thereof, and it is therefore considered that the rubber composition is hardly improved in both of the properties in a well-balanced manner.
In Patent Document 1, as a rubber composition that can be improved in the aforementioned properties in a well-balanced manner, there is described the rubber composition for tires which includes a rubber component containing an isoprene-based rubber and a styrene-butadiene rubber, carbon black and a liquid resin having a softening point of from −20 to 20° C. at a specific compounding ratio.
Also, Patent Document 2 describes the tire including a rubber component containing a diene-based rubber constituted of a modified styrene-butadiene copolymer and a modified conjugated diene-based polymer, and a filler such as carbon black at a specific compounding ratio.
However, any of the tires described in these Patent Documents fail to satisfy the mechanical strength and hardness as well as the rolling resistance performance and processability with a sufficiently high level, and therefore there is still a strong demand for tires that are further improved in these properties.
Meanwhile, Patent Documents 3 and 4 describe a polymer of β-farnesene, but fail to have a sufficient study on practical applications thereof.